The Valid Mosaic Data Region of the CINRAD Network

Along the normal propagation path, the radar ray becomes higher and higher due to effects of the earth curvature and positive elevation angle of transmitter. Actually, radar waves are often blocked as they propagate through mountain areas. When radar beam has occultation, its real sample volume will be smaller than that of zero occultation, echo intensity of targets will be underestimated if it is partial occultation, and targets will be totally missed if complete occultation occurs. It is common practice now to use occultation rate for beam occultation correction. Assuming that radar waves propagate in standard atmosphere, occultation rates of radar beam herein are calculated with the high resolution digital elevation map data, and are utilized to analyze the valid mosaic data region of netted radars and to assess the beam occultation correction. Test shows that echo will be severely underestimated and waves can be considered to be completely blocked when the occultation rate is larger than 55 %. Based on that, in terms of the 14 elevation angles of two Volume Coverage Pattern modes, VCP11 and VCP12, the real detecting ranges of netted radars are obtained. The netted radars include the local CINRAD in Hunan, Jiangxi, Zhejiang, Fujian, Guangdong, Guangxi and Hainan provinces. Compared with the equivalent beam range at the same height, the valid data range of CAPPI is more suitable to represent the real detection range. Among the valid mosaic date regions at the height of 1500 m, 3000 m and 6000 m above the sea level, the largest blank area can be seen in the graph of 1500 m height, but can hardly be seen in that of 6000 m height. In the overlapping area of 6000 m height, most common grids can be detected by three radars or more, some by the maximum of six. Observations in VCP12, compared with that in VCP11 or in VCP21, are more useful to construct mosaic data not only because of its dense vertical sample in low elevation but also its larger valid data range in mountain areas. According to the definition formula of echo intensity, correction of intensities sampled in partial occultation range gates is obtained from its occultation rate. For instance, the correction value is 1.0 dB, 1.5 dB, 2.2 dB, 3.0 dB and 3.5 dB while the occultation rate is 20 %, 30 %, 40 %, 50 % and 55 % respectively. In the experiment of correction, base data are observed in the common latitude longitude grid at the same time by the two weather radars in Wenzhou and Ningbo. The common grids are separated into two groups. The group Ⅰ consists of the grids where Wenzhou radar has partial occultation and Ningbo radar has none. And the group Ⅱ consists of the grids where both two has no occultation. Therefore, the observations of Wenzhou radar are corrected in group Ⅰ, and correction is assessed by the observations of Ningbo radar. The results indicate that the correlative coefficient of corrected data is larger than that of uncorrected in group Ⅰ, and the values are close to that of group Ⅱ, the corrections are effective.